32.2 Mail Header Fields

A header field in the mail buffer starts with a field name at the
beginning of a line, terminated by a colon. Upper and lower case are
equivalent in field names. After the colon and optional whitespace
comes the contents of the field.

You can use any name you like for a header field, but normally
people use only standard field names with accepted meanings.

The ‘From’ header field identifies the person sending the email
(i.e., you). This should be a valid mailing address, as replies are
normally sent there. The default contents of this header field are
computed from the variables user-full-name (which specifies
your full name) and user-mail-address (your email address). On
some operating systems, Emacs initializes these two variables using
environment variables (see General Variables). If this
information is unavailable or wrong, you should customize the
variables yourself (see Easy Customization).

The value of the variable mail-from-style specifies how to
format the contents of the ‘From’ field:

nil

Use just the address, as in ‘king@grassland.com’.

parens

Use both address and full name, as in:
‘king@grassland.com (Elvis Parsley)’.

angles

Use both address and full name, as in:
‘Elvis Parsley <king@grassland.com>’.

any other value

Use angles normally. But if the address must be “quoted” to
remain syntactically valid under the angles format but not
under the parens format, use parens instead. This is
the default.

Apart from ‘From’, here is a table of commonly-used fields:

‘To’

The mailing address(es) to which the message is addressed. To list
more than one address, use commas to separate them.

‘Subject’

The subject of the message.

‘CC’

Additional mailing address(es) to send the message to. This is like
‘To’, except that these readers should not regard the message as
directed at them.

‘BCC’

Additional mailing address(es) to send the message to, which should
not appear in the header of the message actually sent. “BCC” stands
for blind carbon copies.

‘FCC’

The name of a file, to which a copy of the sent message should be
appended. Emacs writes the message in mbox format, unless the file is
in Babyl format (used by Rmail before Emacs 23), in which case Emacs
writes in Babyl format. If an Rmail buffer is visiting the file,
Emacs updates it accordingly. To specify more than one file, use
several ‘FCC’ fields, with one file name in each field.

‘Reply-to’

An address to which replies should be sent, instead of ‘From’.
This is used if, for some reason, your ‘From’ address cannot
receive replies.

‘Mail-reply-to’

This field takes precedence over ‘Reply-to’. It is used because
some mailing lists set the ‘Reply-to’ field for their own
purposes (a somewhat controversial practice).

‘Mail-followup-to’

One of more address(es) to use as default recipient(s) for follow-up
messages. This is typically used when you reply to a message from a
mailing list that you are subscribed to, and want replies to go to the
list without sending an extra copy to you.

‘In-reply-to’

An identifier for the message you are replying to. Most mail readers
use this information to group related messages together. Normally,
this header is filled in automatically when you reply to a message in
any mail program built into Emacs.

‘References’

Identifiers for previous related messages. Like ‘In-reply-to’,
this is normally filled in automatically for you.

The ‘To’, ‘CC’, and ‘BCC’ fields can appear any number
of times, and each such header field can contain multiple addresses,
separated by commas. This way, you can specify any number of places
to send the message. These fields can also have continuation lines:
one or more lines starting with whitespace, following the starting
line of the field, are considered part of the field. Here's an
example of a ‘To’ field with a continuation line:

To: foo@example.net, this@example.net,
bob@example.com

You can direct Emacs to insert certain default headers into the mail
buffer by setting the variable mail-default-headers to a
string. Then C-x m inserts this string into the message
headers. For example, here is how to add a ‘Reply-to’ and
‘FCC’ header to each message: